Robert Siciliano, a member of Boston Children’s Hospital’s charity running team, has been haunted since the bombs exploded at the Boston Marathon — haunted by the victims’ injuries, and by his own behavior.

“I was going right by bloody people and I thought, ‘I can’t just leave them. You’re supposed to help people,’ ” he said. “But I’ve got two little girls. So what the heck do I do?”

The phone went quiet as Siciliano, a security specialist from the North Shore, fought tears. “The decision I made got me to my family” who were at the race, he said. “But it’s still not right with me. I could have done more. Maybe I could have saved a foot.”

In the weeks since the attack, which killed three and injured more than 260, many of the wounded have shared their stories with the public. But in private, some uncounted number of runners and spectators are suffering from feelings of intense guilt because when violence struck, they didn’t dash in to help. Instead, they made sure their own loved ones were safe amid the harrowing chaos — or fled the danger to make sure they would survive to care for their families.

Now, they are talking to therapists, friends, or sometimes only themselves, questioning their split-second decisions. It may be a variation on survivor’s guilt or, as psychologists have said, an attempt to feel some control over a situation in which they were powerless. In any case, the emotions are powerful.

Jonathan Wiggs/Globe Staff

Robert Coughlan wished he had done more to help a visitor find her husband.

“You see all these stories about people who cradled victims’ heads in their hands,” said Chris Pratt, a general contractor from Scituate. He was a few blocks from Copley Square when the bombs exploded, he explained, and he — like the vast majority of people in the area — ran away from the carnage, not toward it.

“People who know me seem to think I’m unselfish,” Pratt said, his voice trailing off. “I’m not down on myself,” he added. “I was so happy to see my family. I’ve got three children [under 4] and a wife who loves me. But I feel that people are worth [helping] even if you don’t know them.”

What makes one bystander jump in and another flee for safety? “There’s no ‘hero gene,’ ” said Jeffrey Pellegrino, a member of the American Red Cross Scientific Advisory Council.

In fact, a large part of a person’s response in such a crisis is not even voluntary.

“When our senses pick up a threat, it goes into the really old part of the brain, and we start producing lots of different chemicals. That’s the fight-or-flight reaction,” Pellegrino explained. “If you are exposed to something novel, like an explosion, your body takes over and it is about self-preservation. There is nothing shameful [about protecting yourself]. Our species wants to continue.”

Once the traumatic event makes its way into working memory, he said, people start to weigh issues. He posed questions that flash through a witness’s mind: “Will the injured person die if I don’t help? What is the benefit for the person if I do help? What’s the benefit for me?”

Bystanders are more likely to respond to an emergency when there’s no one else to help, he added. “As soon as you add more people, you diffuse that responsibility.”

There may not be a “hero gene,” but there is a type, said Jane Blansfield Finch, a clinical social worker and Red Cross volunteer who conducts workshops for clinicians on ethics guiding disaster response. Adventure seekers and risk takers are more apt to instinctively move forward, she said.

Indeed, one of the day’s heroes — former Patriots offensive guard Joe Andruzzi
— spent his football career risking serious injury every time he stepped on the field.

Even though many anointed as heroes said they don’t deserve the honor, the public veneration of those who jump to help in a disaster serves a greater purpose because it encourages altruism, Finch noted. But there’s also a downside. “It puts pressure on people to think, ‘If I’m not out there helping strangers, am I worthwhile?’ ”

But, she explained, “Those who are not trained might naturally — and perhaps sensibly — hold back from doing something because they don’t know what to do or fear doing something wrong.” And, she added, “there is an ethical grounding” for putting oneself, and the needs of loved ones, before the needs of strangers.

Wendy Maeda/Globe Staff

Robert Siciliano, a member of Boston Children’s Hospital’s charity running team, has been haunted by the bombing victims’ injuries, and by his own behavior.

And yet many of the guilt ridden are those who consider themselves precisely the kind of person who would help a stranger. Sylvia Hammerman, a Newton therapist, has a patient who is a nurse and was a few blocks away when the blasts rang out on Boylston Street.

“The next thing she knows, people are running towards her telling her to run the other way, so she did,” Hammerman said. “But upon reflection, she felt bad about her decision. She’s in the health care giving profession, and this is very much at odds with how she thinks of herself.”

Some who wish they had helped said the feelings came over them only later, after the initial shock. In Billerica, Robert Coughlan spent the days after the April 15 attack searching newspapers for a victim with an Hispanic last name — and hoping not to find one.

Coughlan was on the Green Line, at a Boston University stop, when the bombs exploded and MBTA service stopped. Alarmed passengers were told to get off and left to find their own way.

As word spread that there had been an attack, Coughlan started walking toward the finish line with a passenger from Costa Rica. “She had no idea where she was,” he said.

He had planned to help her find her husband, a runner, but then — incredibly — he spotted the very friends he’d been planning to meet up with near Massachusetts Avenue.

His phone battery dead, and worried that in the mayhem he wouldn’t find his pals again, he pointed the woman toward the Prudential Center.

“I have this looming question,” he said. “What happened to them?”

Coughlan, a product and technical support representative for Bose, regularly prays for the couple and beats himself up just as often.

“In retrospect,” he said, “I chose the route of self-preservation over self-sacrifice. And all because of a dead cellphone battery.”

Some of the guilt ridden are dealing with their feelings by increasing their efforts to help others. Lisa Hickey, the mother of a runner who said she searched for her own daughter without stopping for others, is planning to take a first-aid class.

“I’ve never had any experience with a traumatic event like this,” said Hickey, chief executive of the Belmont-based Good Men Project
, an online publication. Hickey — who was so close to the second bomb that shrapnel injured her leg and her ears are still ringing — recalled the devastation all around her. “Someone was doing CPR on someone near me. We passed someone whose leg had been so severely injured you could see the bone.”

With others helping the injured, she felt they were already getting the assistance they needed, she said. “But it’s given me this chance to say, ‘Next time, I’m going to know what to do. I will go towards those people.’ ”

As for Pratt, the Scituate contractor, he plans to run the Marathon next year to raise money for charity. “I think it will help give me some closure,” he said.